New Jersey Gov. Chris Christie announces a partnership between Cooper University Health Care and MD Anderson Cancer Center during a press conference at the State House in Trenton, N.J. June 10, 2013, and Former U.S. President Bill Clinton speaks during the Clinton Global Initiative CGI America meeting in Chicago, Illinois, U.S., on June 13, 2013.

Today's session, called "Cooperation and Collaboration: A Conversation on Leadership," is the product of an odd-couple pairing of Clinton and Christie that has developed in the four years since the popular former president tried unsuccessfully to end Christie's political career before it really took off, sources close to both Clinton and Christie said.

"They're both similar in a lot of ways," one Clinton insider told ABC News. Clinton "respects him and his nature and he likes him as a person. Clinton likes the straightforward style. They have similar backgrounds, they were both prosecutors, Clinton as an attorney general in Arkansas. They're both normal guys from humble beginnings."

The unpredictable Christie was a key fundraiser and campaigner last year for Obama's GOP challenger, Mitt Romney. In return, the first-term Republican governor got to deliver the keynote speech at the Republican National Convention in Tampa.

Then, Superstorm Sandy destroyed much of the Jersey Shore. In the hurricane's aftermath, Christie looked to the federal government to help and, in the process, established a bipartisan working relationship with Obama that infuriated GOP stalwarts.

Now, as Christie faces re-election and is touted as a possible contender for Clinton's old office on Pennsylvania Avenue, Christie is sharing the spotlight with him, a man who was once impeached because of Christie's own GOP.

Representatives of both men declined to comment for this story, saying Clinton and Christie will say what needs to be said when they take the stage at today's Clinton Global Initiative event.

Intimates of both men say the real story behind today's event is the private friendship that has developed between the ex-president and a governor who would like to be president. The two have met, traded war stories, talked on the phone, all inside the four years since Clinton campaigned hard on behalf of then-Gov. Jon Corzine, the incumbent Christie beat in 2009.

People familiar with the private conversations between the governor and the former president said this is the second time Clinton invited Christie to play a starring role at the Clinton Global Initiative in Chicago. Christie had been tentatively scheduled to attend last year but was scrubbed from the final agenda.

In other words, this particular "bromance," they said, has been going on longer than Christie's new, high-profile attachment to the latest inhabitant of the White House, Obama.

Insiders say the unlikely pair of Clinton and Christie started with Clinton's taking note of Christie's political acumen even as the ex-president was working against Christie. At the time, Clinton was crisscrossing New Jersey to aid Corzine's floundering re-election bid.

As he worked crowds across the Garden State, Clinton painted Christie, 50, as hostile toward women, the middle class and working people while he openly questioning "why are we having an election?"

Clinton, 66, insisted that the poor economy was making people think ill of Corzine, and that he needed to travel New Jersey because "It matters what the governors do, it matters what the states do," so voters needed to send Corzine back to Trenton.

Despite the speeches, Clinton had started noticing Christie's tough talk and skill at sparring with reporters, sources said. Soon afterward, Christie again impressed the always-a-politician Clinton as the rookie governor battled New Jersey's powerful teachers unions.

So, later in 2010, when the two wound up a few rows away from each other at the Big East basketball tournament at Madison Square Garden, Clinton had his people arrange for a quick chat with Christie. That led to a summit of sorts at
Clinton's post-presidential office in Harlem, where Clinton and Christie talked politics and public policy.

"It was very good, long Clintonesque conversation," one Clinton person said.

What was clear during that meeting and the subsequent opportunities for the two pols to talk is how much alike Clinton and Christie are.

"Christie is like Clinton," the insider said. "You couldn't be in a room with Chris Christie and not come away thinking, 'Christie is a neat guy to be around. He's normal. He grabs a room. Like Clinton.'"

Never a darling of the Republican right-wing, Christie and his aides are well aware that a trip to the Clinton conference is more than just a pleasant diversion to Chicago. It can, in fact, provide potential opponents with more ammunition to demonstrate how disloyal Christie is to the conservative electorate that turns out in presidential primaries.

For a governor who constantly reminds people he can work with leaders of all stripes, the appearance today and the friendship with Clinton are worth the political risks.

"This is a stature thing," a Christie source said. "It burnishes his [Christie's] bipartisan credentials and it shows the governor is a serious guy. He's talking about leadership with Clinton. That's a big deal."

As for Clinton's efforts to destroy Christie last time around: Christie, the source said, "understands the game as well as anybody. The campaign's over."